Heat the oven to 200C/180C fan/gas 6. Tip the flours into a bowl with the bicarb, onion flakes, nigella seeds and 1½ tsp salt. Stir together and make a well in the middle. Tip in the butter and yogurt, and bring together into a soft, sticky dough using a spatula. If it seems dry, add a splash of milk.
Tip the dough out onto a lightly floured surface and gently form into a round. Lift onto a lightly floured baking sheet, cut a deep cross in the top using a sharp knife and sprinkle over the oats. Bake for 1 hr, or until golden and the loaf sounds hollow when tapped underneath. Leave to cool on a wire rack.
While the bread cools, beat the butter, mustard and a pinch of sea salt together until light and airy. Spoon into a small pot. Slice the bread and serve with the mustard butter for spreading over.
